It is now well known that various fibers can be used in the reinforcement of cementitious products. One of the more well-known reinforced cementitious products is cement reinforced with asbestos fibers. The asbestos fibers are combined with cement in the form of a built-up laminate to provide a reinforced product, such as cement pipes and cement sheets or boards and the like, having good strength characteristics.
In the manufacture of such asbestos fiber reinforced cementitious products, two processes are well known to those skilled in the art. The first is the so-called Hatschek process for the production of reinforced cementitious pipe and the second is the Magniani process for the production of boards formed of reinforced cement. In both of the processes, asbestos-fibers are mixed with a cement slurry to form a pulp, and then the pulp is placed onto a foraminous forming member (a cylinder in the case of the Hatschek process and a flat, usually endless, belt in the Magniani process). Moisture from the slurry is removed by applying suction thereto whereby water is drawn through the foraminous forming member.
The mechanism underlying the effectiveness of asbestos fibers in the manufacture of reinforced cementitious products is not fully understood at the present time. The asbestos reinforcement appears to maintain, to at least some degree, the retention of water as the reinforced cement product is being manufactured to prevent excessive dehydration which would cause the cement product to crumble.
It has been hypothesized that the high surface charge density of asbestos fibers makes them highly reactive to retain small cement particles along with water to prevent cement from being carried off with the water during dehydration on the foraminous support. That high reactivity is accentuated by the fact that the asbestos fibers have a high specific surface area (viz., of the order of 10-20 m.sup.2 /g). Thus, the highly reactive surfaces of the asbestos fibers are believed to flocculate the cement and retain it to provide a reinforced cement product having good structural strengths.
Various attempts have been made to omit asbestos from such reinforced cementitious products but without success. In the absence of the asbestos fibers dispersed in the cementitious material, the rate at which water can be removed so that the cementitious product can be cured is significantly reduced as a result of excessive hydration.
It has been proposed, in French Patent No. 2,317,250, to partially replace asbestos fibers with glass fibers. Even that technique has not met with any appreciable success. Glass fibers, when combined with cementitious materials in the manufacture of reinforced cementitious products have a tendency to adhere together, remaining in bundles, thereby disturbing the rate at which water can be removed through the foraminous forming member. In general, the presence of glass fibers in such reinforced cement products makes such products, in the hydrated state, too porous and causes the water present in the cement slurry to be removed too rapidly, carrying with it large quantities of the cement itself. Because glass fibers have quite low surface areas (of the order of 0.1-0.2 m.sup.2 /g), they do not share in the ability of asbestos to retain either cement or water. Thus, it has not been possible, up to the present invention, to form, on a Hatschek machine, reinforced cement products containing more than 2% by weight of glass fibers.